RU2789633C1 - Method for determining the temperature of the beginning of crystallization of liquid hydrocarbons and fuels for jet engines - Google Patents

Abstract

FIELD: quality control.

SUBSTANCE: invention relates to the field of quality control of liquid hydrocarbons and fuels for jet engines and can be used for express determination of the temperature of the beginning of crystallization. According to the claimed method, the determination of T_bc consists in sampling, measuring the current value of the specific heat flux from temperature at a constant rate with a cooling and heating rate equal to 5°C/min, followed by the construction of a graphical dependence in the coordinates “heat flux-temperature”. On the resulting graphical dependence, the temperature range in which the temperature of the beginning of crystallization is located is fixed and converted into a function in Fourier coordinates, from which the acceleration of the change in the rate of heat flux release during cooling is found, where, in the established temperature range, the temperature of the first minimum after the minimum extreme is taken as the temperature of the beginning of crystallization.

EFFECT: increase in the information content and a reduction in the complexity of determining the temperature of the beginning of crystallization of liquid individual hydrocarbons and fuels for jet engines while maintaining accuracy requirements.

1 cl, 2 dwg, 2 tbl

Description

The invention relates to methods for monitoring the quality of liquid hydrocarbons and fuels for jet engines, in particular to methods for determining the temperature of the onset of crystallization (T _nk ) using the method of differential scanning calorimetry and can be used at refineries in the production of fuels, in fuel quality control laboratories, in oil product supply organizations, as well as in research work.

The authors were faced with the task of developing a method that allows, with increased information content and reduced labor intensity, to determine the temperature of the onset of crystallization of liquid hydrocarbons and jet fuels, the determination of which corresponds to the methods used, using the method of differential scanning calorimetry.

Currently, the main low-temperature indicator characterizing the formation of the first hydrocarbon crystals is the crystallization start temperature, that is, the temperature at which the formation of a solid phase begins in the volume of the analyzed sample freezing - p. 2).

The essence of this method lies in the visual control of the state of aggregation of the analyzed sample during its cooling. The sample is subjected to cooling in such a way that the temperature of the cooling mixture is constantly 15 ± 2°C lower than the temperature of the test fuel in the test tube with double walls and a stirrer, and 5°C before the expected T _NK , the test tube is removed from the bath, quickly lowered into a beaker with alcohol and observe the condition of the tested fuel. In the absence of crystals in the fuel, the test tube is again lowered into the bath and further observations are carried out every degree, observing the duration of a single observation of no more than 12 s. The discrepancy between two successive determinations should not exceed 1°C.

In the standard method for estimating the temperature of the onset of crystallization of fuels for aviation gasolines and jet fuels, when the sample is cooled, the cooling rate is maintained from 0.1°C/min to 0.5°C/min, which leads to a significant duration of one test (1 - GOST 5066 -2018 Motor fuels.Methods for determining the cloud point, the beginning of crystallization and freezing - p. 5).

The temperature of the beginning of crystallization of individual hydrocarbons is determined by graphical methods when analyzing the obtained dependences of the cooling temperature on the length of time, or using Bauman-From and Zhukov devices (2 - GOST 18995.5-73. Organic chemical products - p. 5-6). Limitations in the temperature range of the study range from minus 50°C to 250°C.

The essence of the method for determining _Tnc of individual hydrocarbons (IHC) is to monitor the temperature change of the cooled liquid or molten IHC in time and establish _Tnc by a graphical method (2 - GOST 18995.5-73. Organic chemical products - pp. 5-6). Highly pure substances and technical products, except for benzene, are subjected to cooling in such a way that the temperature of the cooling mixture is constantly 3-5 ° C lower than the expected crystallization temperature, which is a fundamental difference from the standard for determining the temperature of the onset of crystallization of fuels for aviation gasoline, jet and diesel fuels.

Common disadvantages of the known methods are:

- insufficient accuracy and reliability, especially at low temperatures;

- the variability of the rate of cooling and heating, as well as the impossibility of its control;

- the duration of the test.

The thermal method - differential scanning calorimetry (DSC) - is a topical and rapidly developing method for studying low-temperature properties in our country. This method has a number of advantages over standardized methods, in particular, an extended test temperature range (from minus 150°C to 500°C).

The closest in technical essence is a method for measuring the specific heat flux (effect in the source) during thermal analysis using calorimeters (3 - SU Patent No. 276490 G01N 25/20, 1968 - prototype). Determining the phase transition start temperature includes: sampling, measuring the current value of the specific heat flux versus temperature at a given rate of successive cooling and heating, plotting a graphical dependence in the “heat flux-temperature” coordinates, fixing the start point of the release of the specific heat flux, beyond which take the temperature corresponding to the beginning of the phase transition from liquid to solid state.

Despite the use of differential scanning calorimetry, this method has its drawback. An indirect method for determining the crystallization onset temperature equates the onset of specific heat flux release to the crystallization onset temperature, which leads to a scatter in the determined indicator and reduces accuracy.

This fact indicates the need to use additional ways to use the research method, since this discrepancy in the determined indicator arises due to fundamental differences in the conditions of the test (4 - Scientific and technical report on research, "code Intervision-21". - M: FAA " 25 State Research Institute of Chemmotology of the Ministry of Defense of the Russian Federation, 2021. Inv. No. 4736).

The technical result is an increase in information content and a decrease in the complexity of determining the temperature of the onset of crystallization of liquid individual hydrocarbons and jet fuels while maintaining accuracy requirements.

The specified technical result is achieved by the fact that in a known method, determining the temperature of the onset of crystallization of liquid hydrocarbons and jet fuels, including sampling, measuring the current value of the specific heat flux from temperature at a given rate of sequential cooling to a temperature below the expected temperature of the onset of crystallization and heating, building graphic dependence in the coordinates "heat flow-temperature", fixing the point of the beginning of the release of the specific heat flow, corresponding to the temperature of the beginning of the phase transition from liquid to solid state, according to the invention, cooling and heating of the sample is carried out at a constant rate of 5 ° C / min to a temperature of 10 °C below the expected temperature of the beginning of crystallization, on the graphical dependence of the change in the phase transition from solid to liquid state, the end point of the absorption of the heat flux at a specific temperature is fixed, according to which the point on the line is determined. and cooling with the corresponding temperature, the segment between these points is taken as the temperature range, which is converted into a function in Fourier coordinates, the acceleration of the change in the rate of heat flux release during cooling is found and the minimum extremum point is fixed on the obtained graphical dependence, and the temperature is taken as the crystallization onset temperature the first minimum after the minimum extremum.

The essence of the invention when achieving the specified technical result lies in the fact that the cooling and heating of the sample is carried out at a constant rate of 5°C/min to a temperature of 10°C below the expected crystallization start temperature.

Increasing the information content is achieved by expanding the temperature limits of the study of liquid hydrocarbons and jet fuels from 500°C to minus 150°C. (Passport for differential scanning calorimeter, model DSC 3. "Mettler-Toledo GmbH", Switzerland. 2019.)

The essence of the claimed method is illustrated by graphical dependencies (Fig. 1 and Fig. 2), which shows the change in the specific heat flux for fuel and for IUV, respectively, coming from the sample when it is cooled and obtained by the sample when it is heated. The tests are carried out using the DSK-3 device (Passport for a differential scanning calorimeter, model DSC 3. "Mettler-Toledo GmbH", Switzerland. 2019.). Graphic dependences are received in the specialized software provided for the given calorimeter: "STAR'e Software").

In FIG. 1-1 shows a graphical dependence of the specific heat flow of fuel for a jet engine on temperature, obtained at a constant rate of 5 ° C / min of cooling (curve "a") and heating (curve "b)));

fig. 1-2 shows a graphical dependence of the function of the analyzed curve in Fourier coordinates;

fig. 1-3 shows the graphic dependence of the second derivative of the acceleration of the change in the rate of heat flux release during the cooling process, indicating the determined T _nk .

In FIG. 2-1 shows a graphical dependence of the specific heat flux of an individual hydrocarbon (n-octane) on temperature, obtained at a constant rate of 5°C/min cooling (curve "a") and heating (curve "b");

fig. 2-2 shows a graphical dependence of the function of the analyzed curve in Fourier coordinates;

fig. 1.-3 shows the graphical dependence of the second derivative of the acceleration of the change in the rate of heat flux release during the cooling process, indicating the determined T _nk .

The method is implemented as follows.

Example #1. It is necessary to determine the crystallization temperature of jet fuel using the method of differential calorimetry. The selected fuel sample for determining T _nk by the claimed method is the state standard sample (GSO) TNK-PA is a jet fuel corresponding to GOST 10227 (6 - GOST 10227-2013 Jet fuels), class RT. The certified value of the temperature of the beginning of crystallization, in accordance with GOST 5066 (1 - GOST 5066-2018 Motor fuels. Methods for determining the cloud point, the beginning of crystallization and freezing) is minus 60 ° C, the limits of the absolute error of the certified GSO value (P = 0.95) are ± 0.4°C.

To determine the crystallization onset temperature, the sample is placed in a 40 µl aluminum crucible, weighed on an analytical balance, and the sample weight (9.65 mg) is fixed, after which the crucible is hermetically sealed with a lid and placed on one of the heat flux sensors of the differential scanning calorimeter, on the other the sensor is placed in an empty sealed crucible. Further, in accordance with the given program, i.e. depending on the properties of the analyzed sample (test name, sample weight, temperature range of the study, the rate of change in the cooling-heating temperature), tests are carried out to determine the crystallization onset temperature. Graphical dependences of changes in the specific heat flux on temperature are carried out with a constant cooling and heating rate equal to 5°C/min (Fig. 1-1, curve "a" and "b"). The sample is preliminarily tested at cooling and heating rates of ± 15°C/min. This experiment is not considered further.

On the obtained thermoanalytical curve "a", obtained at a cooling rate of 5 ° C / min, the temperature range of the study is determined, in which the temperature of the beginning of crystallization is located, and which is concluded from the end point of the absorption of the heat flux at a specific temperature (point "A"), according to which determines the point on the cooling line with the corresponding temperature (point "C") and up to the temperature of the beginning of absorption of the specific heat flux (point "B"), this range corresponds to the boundaries from "C" to "B". Using the STAR'e Software allows you to determine the state of the Fourier curve function obtained under the following conditions: the calculation depth is 1, the reduction factor is 1, the period is 60 (Fig. 1-2). Then, to clarify the temperature of the beginning of crystallization from the obtained Fourier curve, a graphical dependence of the second derivative on temperature is obtained under the following conditions: the order of the function is 2, the number of points is 25 (Fig. 1-3). On the obtained dependence of Fig. 1-3 fix the point of the minimum extremum in the studied temperature range, from point "C" to "B", and which is highlighted by a black dotted line. The temperature of the beginning of crystallization determined by the claimed method corresponds to the temperatures of the first minimum after a certain minimum extremum, which is in the temperature range, which is highlighted by a dotted line in Fig. 1-3. The obtained value of T _NK for GSO TNK-PA according to the claimed method is minus 60.19°C, which corresponds to the certified value for GSO (minus 60±0.4°C).

Example #2. Following a similar sequence of testing by the claimed method, the crystallization onset temperature for an individual hydrocarbon (n-octane) was determined. The result of the test and the graphic dependence, according to which the crystallization start temperature is determined, are shown in Fig. 2-1 and FIG. 2-3.

As a result of determining T _NK by the claimed method, the obtained value was minus 51.87°C, which corresponds to the temperature determined in accordance with GOST 18995.5 (minus 52°C).

The declared method was used to determine the temperature of the onset of crystallization for other jet fuels: RT, T-6, T-1 and TS-1. The results of the determination are shown in table 1.

From table 1 it can be seen that the values of T _NK obtained by the claimed method for all compared samples of the deviation are within the permissible errors.

The claimed set of essential features of the method set forth in the claims, the authors did not identify from the sources of patent and scientific and technical information, for which a comparative analysis of the known DSC method and the proposed method was carried out, the results of which are presented in table 2, which allows us to consider the technical solution that meets grounds for the conditions of patentability: novelty, inventive step and industrial applicability.

Thus, the application of the invention will increase the information content and reduce the complexity of determining the temperature of the onset of crystallization of liquid hydrocarbons and petroleum products in the temperature range of the study (from minus 150°C to 500°C). The test conditions, namely the cooling/heating rate equal to ±5°C/min and the reduced error of the DSC method (±0.2°C), will reduce the time of the test without losing the information content of the determined low-temperature indicator.

Claims (1)

A method for determining the crystallization onset temperature of liquid hydrocarbons and jet fuels, including sampling, measuring the current value of the specific heat flux from temperature at a given rate of sequential cooling to a temperature below the expected temperature of the onset of crystallization and heating, plotting a graphical dependence in the heat flux-temperature coordinates ”, fixing the start point of the release of the specific heat flux corresponding to the temperature of the beginning of the phase transition from liquid to solid state, characterized in that the cooling and heating of the sample is carried out at a constant rate of 5 ° C / min to a temperature of 10 ° C below the expected temperature of the onset of crystallization, on the graphic dependence of the change in the phase transition from a solid to a liquid state, the end point of the absorption of the heat flux at a specific temperature is fixed, by which the point on the cooling line with the corresponding temperature is determined, the segment between these points are taken as the temperature range, which is converted into a function in Fourier coordinates, the acceleration of the change in the rate of heat flux release during cooling is found and the point of the minimum extremum is fixed on the obtained graphic dependence, and the temperature of the first minimum after the minimum extremum is taken as the temperature of the onset of crystallization.

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